The use of hydrogels as drug delivery carriers has been explored since the beginning of the controlled release era in the 1960's primarily focusing on polymer hydrogels and animal-derived biomaterials. However, hydrogels consisting of synthetic polymers do not represent an ideal system for biomedical applications due to: (i) component and degradation product toxicity (e.g., many polymers require the use of toxic cross-linkers, like glutaraldehyde, and other chemicals that pose a life threat whereas others such as polyglycolic-polylactic acid and its analogues during degradation release acids locally), (ii) post-gelation polymer swelling often causes pain in the host, and (iii) release of the active compound over brief periods of time due to the large pores of the polymer network. Furthermore, animal-extracted biopolymers such as collagen, gelatin, fibrin, and laminin [1,2,3,4] are not considered in real-life, clinical applications involving humans due to their origin and the risk of inflammatory host response from viruses, bacteria, and other unknown substances that may be present in the donor tissue. In response to the need of biocompatible drug release systems, biodegradable synthetic polymers were developed [5,6,7,8]. Despite the extensive research and the constant development of novel hydrogel systems, these challenges have not been completely resolved yet. 1 F. A. Kincl, L. A. Ciaccio, S. B. Henderson, Sustained Release Preparations, XVI. Collagen as a Drug Carrier, Archiv. der Pharmazie 317 (1984) 657-661.2 K. H. Stenzel, T. Miyata, A. L. Rubin, Collagen as a biomaterial, Annu. Rev. Biophys. Bioeng. 3 (1974) 231-253.3 A. L. Rubin, K. H. Stenzel, T. Miyata, M. J. White, M. Dunn, Collagen as a vehicle for drug delivery, J. Clin. Pharmacol. 13 (1973) 309-312.4 F. Greco, L. Depalma, N. Spagnolo, A. Rossi, N. Specchia, A. Gigante, Fibrin antibiotic mixtures—an invitro study assessing the possibility of using a biologic carrier for local-drug delivery, J. Biomed. Mater. Res. 25 (1991) 39-51.5 J. Folkman, D. M. Long, The use of silicone rubber as a carrier for prolonged drug therapy, J. Surg. Res. 4 (1964) 139-142.6 S. J. Desai, A. P. Simonelli, W. I. Higuchi, Investigation of factors influencing release of solid drug dispersed in inert matrices, J. Pharm. Sci. 54 (1965) 1459-1464.7 B. K. Davis, Control of diabetes with polyacrylamide implants containing insulin, Experientia 28 (1972) 348.8 R. Langer, J. Folkman, Polymers for sustained-release of proteins and other macromolecules, Nature 263 (1976) 797-800.
Previously, a nanofiber hydrogel consisting of the self-assembling peptide ac-(RADA)4-CONH2 (SEQ ID NO: 3) (where R is arginine, A is alanine and D is aspartic acid) was studied for controlled release of small, model-drug molecules [9]. In a recent study, it was shown that proteins with different molecular weights and isoelectric points were slowly released through the ac-(RADA)4-CONH2 (SEQ ID NO: 3) peptide hydrogel and the release kinetics were studied over a period of 3 months [10]. Self-assembling peptide hydrogels are injectable because they can be formed inside the body upon interaction of the peptide solution with biological fluids. Upon being introduced to electrolyte solutions, self-assembling peptides form nanofibers with diameters between 10 nm-20 nm, which are further organized to form a scaffold hydrogel containing water up to ˜99.5% (w/v) and form pore with sizes between 5 nm-200 nm in diameter [11]. Peptide gelation does not require harmful materials, such as toxic cross-linkers, to initiate the sol-gel transition while the degradation products of the hydrogel are natural amino acids, which can be metabolized and reused by the body. The fact that the sol-gel transition occurs at physiological conditions facilitates mixing of the peptide solution with bioactive molecules and co-injection in a tissue-specific manner to form the drug delivery vehicle in the tissue. Peptide scaffold hydrogels are biocompatible, amenable to molecular design, and have been used in a number of tissue engineering applications including bone and cartilage reconstruction, neuronal and heart tissue regeneration, wound healing, angiogenesis, and haemostasis [12,13]. Self-assembling peptide hydrogels provide a platform that makes them ideal for a wide range of bionanomedical applications as they facilitate cell migration inside the hydrogel. Furthermore they are non-toxic, non-immunogenic, non-thrombogenic, biodegradable, and applicable to localized therapies through injection to a particular tissue [14,15]. 9 Y. Nagai, L. D. Unsworth, S. Koutsopoulos, S. Zhang Slow release of molecules in self-assembling peptide nanofiber scaffold, J. Control. Rel. 115 (2006) 18-25.10 S. Koutsopoulos, L. D. Unsworth, Y. Nagai, S. Zhang, Controlled release of functional proteins through designer self-assembling peptide nanofiber hydrogel scaffold, Proc. Natl. Acad. Sci. U.S.A. 106 (2009) 4623-4628.11 S. G. Zhang, T. Holmes, C. Lockshin, A. Rich, Spontaneous assembly of a self-complementary oligopeptide to form a stable macroscopic membrane, Proc. Natl. Acad. Sci. U.S.A. 90 (1993) 3334-3338.12 S. Koutsopoulos, S. Zhang, Three-dimensional neural tissue cultures in biomimetic hydrogel scaffolds consisting of self-assembling peptides. Proc. Natl. Acad. Sci. U.S.A. (2012) (submitted).13 J. Kisiday, M. Jin, B. Kurz, H. Hung, C. Semino, S. Zhang, A. J. Grodzinsky, Self-assembling peptide hydrogel fosters chondrocyte extracellular matrix production and cell division: Implications for cartilage tissue repair, Proc. Natl. Acad. Sci. U.S.A. 99 (2002) 9996-10001.14 M. E. Davis, J. P. M. Motion, D. A. Narmoneva, T. Takahashi, D. Hakuno, R. D. Kamm, S Zhang, R. T. Lee, Injectable self-assembling peptide nanofibers create intramyocardial microenvironments for endothelial cells, Circulation 111 (2005) 442-450.15 R. G. Ellis-Behnke, Y.-X. Liang, S.-W. You, D. K. C. Tay, S. Zhang, K.-F. So, G. E. Schneider, Nano neuro knitting: peptide nanofiber scaffold for brain repair and axon regeneration with functional return of vision, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 5054-5059.
Large proteins such as antibodies are larger and more complex than traditional organic and inorganic drugs due to the presence of multiple functional groups in addition to complex three-dimensional structures, and their formulation for sustained release poses difficult challenges. In order for the antibody to remain biologically active the formulation must protect the functional properties of the antibody for the duration of the therapy. There are multiple pathways for the antibody to degrade during sustained release due to the loss of three dimensional structure or chemical instability. These challenges are attenuated by the length of the therapy. The presentation of functional antibodies with therapeutic properties is important for sustained delivery biomedical applications. As such, there is a need to develop a sustained release system based on biodegragable peptides that can provide delivery of therapeutically useful large proteins such as an antibody for extended periods of time.